Network Working Group A. Keromytis
Request for Comments: 2857 University of Pennsylvania
Category: Standards Track N. Provos
Center for Information Technology Integration
June 2000
The Use of HMAC-RIPEMD-160-96 within ESP and AH
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This memo describes the use of the HMAC algorithm [RFC 2104] in
conjunction with the RIPEMD-160 algorithm [RIPEMD-160] as an
authentication mechanism within the revised IPSEC Encapsulating
Security Payload [ESP] and the revised IPSEC Authentication Header
[AH]. HMAC with RIPEMD-160 provides data origin authentication and
integrity protection.
Further information on the other components necessary for ESP and AH
implementations is provided by [Thayer97a].
1. Introduction
This memo specifies the use of RIPEMD-160 [RIPEMD-160] combined with
HMAC [RFC 2104] as a keyed authentication mechanism within the
context of the Encapsulating Security Payload and the Authentication
Header. The goal of HMAC-RIPEMD-160-96 is to ensure that the packet
is authentic and cannot be modified in transit.
HMAC is a secret key authentication algorithm. Data integrity and
data origin authentication as provided by HMAC are dependent upon the
scope of the distribution of the secret key. If only the source and
destination know the HMAC key, this provides both data origin
authentication and data integrity for packets sent between the two
parties; if the HMAC is correct, this proves that it must have been
added by the source.
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RFC 2857 HMAC-RIPEMD-160-96 within ESP and AH June 2000
In this memo, HMAC-RIPEMD-160-96 is used within the context of ESP
and AH. For further information on how the various pieces of ESP -
including the confidentiality mechanism -- fit together to provide
security services, refer to [ESP] and [Thayer97a]. For further
information on AH, refer to [AH] and [Thayer97a].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
2. Algorithm and Mode
[RIPEMD-160] describes the underlying RIPEMD-160 algorithm, while
[RFC 2104] describes the HMAC algorithm. The HMAC algorithm provides
a framework for inserting various hashing algorithms such as RIPEMD-
160.
HMAC-RIPEMD-160-96 operates on 64-byte blocks of data. Padding
requirements are specified in [RIPEMD-160] and are part of the
RIPEMD-160 algorithm. Padding bits are only necessary in computing
the HMAC-RIPEMD-160 authenticator value and MUST NOT be included in
the packet.
HMAC-RIPEMD-160-96 produces a 160-bit authenticator value. This
160-bit value can be truncated as described in RFC2104. For use with
either ESP or AH, a truncated value using the first 96 bits MUST be
supported. Upon sending, the truncated value is stored within the
authenticator field. Upon receipt, the entire 160-bit value is
computed and the first 96 bits are compared to the value stored in
the authenticator field. No other authenticator value lengths are
supported by HMAC-RIPEMD-160-96.
The length of 96 bits was selected because it is the default
authenticator length as specified in [AH] and meets the security
requirements described in [RFC 2104].
2.1 Performance
[Bellare96a] states that "(HMAC) performance is essentially that of
the underlying hash function". [RIPEMD-160] provides some
performance analysis. As of this writing no detailed performance
analysis has been done of HMAC or HMAC combined with RIPEMD-160.
[RFC 2104] outlines an implementation modification which can improve
per-packet performance without affecting interoperability.
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RFC 2857 HMAC-RIPEMD-160-96 within ESP and AH June 20003. Keying Material
HMAC-RIPEMD-160-96 is a secret key algorithm. While no fixed key
length is specified in [RFC 2104], for use with either ESP or AH a
fixed key length of 160-bits MUST be supported. Key lengths other
than 160-bits SHALL NOT be supported. A key length of 160-bits was
chosen based on the recommendations in [RFC 2104] (i.e. key lengths
less than the authenticator length decrease security strength and
keys longer than the authenticator length do not significantly
increase security strength).
[RFC 2104] discusses requirements for key material, which includes a
discussion on requirements for strong randomness. A strong pseudo-
random function MUST be used to generate the required 160-bit key.
Implementors should refer to RFC 1750 for guidance on the
requirements for such functions.
At the time of this writing there are no specified weak keys for use
with HMAC. This does not mean to imply that weak keys do not exist.
If, at some point, a set of weak keys for HMAC are identified, the
use of these weak keys must be rejected followed by a request for
replacement keys or a newly negotiated Security Association.
[ESP] describes the general mechanism to obtain keying material for
the ESP transform. The derivation of the key from some amount of
keying material does not differ between the manual and automatic key
management mechanisms.
In order to provide data origin authentication, the key distribution
mechanism must ensure that unique keys are allocated and that they
are distributed only to the parties participating in the
communication.
[RFC 2104] states that for "minimally reasonable hash functions" the
"birthday attack" is impractical. For a 64-byte block hash such as
HMAC-RIPEMD-160-96, an attack involving the successful processing of
2**64 blocks would be infeasible unless it were discovered that the
underlying hash had collisions after processing 2**30 blocks. (A
hash with such weak collision-resistance characteristics would
generally be considered to be unusable.) No time-based attacks are
discussed in the document.
While it it still cryptographically prudent to perform frequent
rekeying, current literature does not include any recommended key
lifetimes for HMAC-RIPEMD. When recommendations for HMAC-RIPEMD key
lifetimes become available they will be included in a revised version
of this document.
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RFC 2857 HMAC-RIPEMD-160-96 within ESP and AH June 20004. Interaction with the ESP Cipher Mechanism
As of this writing, there are no known issues which preclude the use
of the HMAC-RIPEMD-160-96 algorithm with any specific cipher
algorithm.
5. Security Considerations
The security provided by HMAC-RIPEMD-160-96 is based upon the
strength of HMAC, and to a lesser degree, the strength of RIPEMD-160.
At the time of this writing there are no known practical
cryptographic attacks against RIPEMD-160.
It is also important to consider that while RIPEMD-160 was never
developed to be used as a keyed hash algorithm, HMAC had that
criteria from the onset.
[RFC 2104] also discusses the potential additional security which is
provided by the truncation of the resulting hash. Specifications
which include HMAC are strongly encouraged to perform this hash
truncation.
As [RFC 2104] provides a framework for incorporating various hash
algorithms with HMAC, it is possible to replace RIPEMD-160 with other
algorithms such as SHA-1. [RFC 2104] contains a detailed discussion
on the strengths and weaknesses of HMAC algorithms.
As is true with any cryptographic algorithm, part of its strength
lies in the correctness of the algorithm implementation, the security
of the key management mechanism and its implementation, the strength
of the associated secret key, and upon the correctness of the
implementation in all of the participating systems. [Kapp97]
contains test vectors and example code to assist in verifying the
correctness of HMAC-RIPEMD-160-96 code.
6. Acknowledgements
This document is derived from work by C. Madson and R. Glenn and from
previous works by Jim Hughes, those people that worked with Jim on
the combined DES/CBC+HMAC-MD5 ESP transforms, the ANX bakeoff
participants, and the members of the IPsec working group.
7. References
[RIPEMD-160] 3.ISO/IEC 10118-3:1998, "Information technology -
Security techniques - Hash-functions - Part 3:
Dedicated hash-functions," International Organization
for Standardization, Geneva, Switzerland, 1998.
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RFC 2857 HMAC-RIPEMD-160-96 within ESP and AH June 20009. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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